Fluorescent nanoparticles have tremendous promise as indicators and photon sources of biotechnological and information applications such as biological imaging, sensor technology, microarrays, and optical computing. These applications require size-controlled, monodisperse, bright nanoparticles that can be specifically conjugated to biological macromolecules or arranged in higher-order structures.
Two general approaches have emerged in recent years for synthesizing highly fluorescent, water-soluble nanoparticles for use in a range of demanding biological and analytical applications. In the first approach, the nanoparticulate material itself is fluorescent (such as semiconductor nanocrystals or metal nanocrystals). In the second, the fluorescent nanoparticles are based on the incorporation of organic dye molecules. Given the vast diversity of organic dye molecules and the exquisite sensitivity of these dye molecules to their local environment, the latter approach raises the possibility of developing nanoparticles with a broad range of precisely controlled fluorescence characteristics.
Based on the work of Stöber et al., silica nanoparticles with an embedded dye have been synthesized in a range of sizes, colors, and architectures. In all previous reports of photophysical properties, the dye which is covalently bound inside the silica particle is observed to be quenched in comparison to the free dye. However, in the case of poly(organosiloxane) microgels in which the dye is non-covalently attached and loaded through diffusion, a slight increase in fluorescence efficiency is observed. For other materials, for example polystyrene microspheres, the quantum efficiency of the embedded dye is the same or less than the free dye. The quenching of fluorescence is usually attributed to either intraparticle energy transfer or non-radiative decay into the silica matrix. Both of these pathways are likely influenced by the local dye environment within the particle, suggesting that precise control of the architecture within the particle might ameliorate quenching or even lead to fluorescence enhancement.
There is still a need for highly fluorescent nanoparticles less than 30 nm with covalently attached organic dyes.